We've done a few things like this over the years.
First (and most mildly I suppose) I ran a 3.0 campaign that was my first 3e campaign for about a year. Then, 2 or 3 years later, we decided to revisit that campaign and we converted to 3.5.
Sometime after that I wanted to "test drive" a couple systems for the group to see how they liked them. The players made Risus characters (which is incredibly easy, fast and rules light) and played the first half of the adventure. Then I took their characters and converted them over to Savage Worlds and they played the second half of the adventure on subsequent sessions. Actually worked out extremely well due to the way that Risus characters are conceptualized as Cliches, which were then very easy to visualize as SW characters once I saw how the players played them. Ultimately the rest of the group decided that they liked SW and were lukewarm about Risus (though I still think that Risus is absolutely awesome for some types of games).
The other thing we did was to use the Warhammer Fantasy Battles rules for playing out large conflicts when we were playing WFRP2. The GM had a lot of familiarity with the wargame rules and he structured the campaign so that there were 3 or 4 such battles over the course of it. Was a of fun and nice change of pace on the nights we did the battles.
Similarly when I ran WFRP2 as a "Warhammer Pirates of the Caribbean" game, we used the Pirates of the Spanish Main ships as minis for the "naval combat mini-game" that I designed to go with that system. It was pretty fast and easy for most of the encounters we used it for. Maybe took 20 minutes of ship to ship battle before the boarding action started and we reverted to regular WFRP combat. And we had one or two big naval battles that took maybe an hour to hour and a half to resolve.
Overall I'd say that my experience with different systems tossed in as a change of pace have been very positive.